Small sacs of air in the lungs are called which of the following?

• A. Lipids
• B. Pharynx
• C. Bronchioles
• D. Alveoli

Answer: (D)

Gas exchange in the lungs occurs where air meets blood—in tiny, balloon-like sacs called alveoli. These alveoli sit at the ends of the airways and are surrounded by a dense network of capillaries. Their walls are extremely thin and coated with a thin liquid film, and the vast total surface area allows oxygen to diffuse into the blood while carbon dioxide diffuses out to be exhaled. Lipids are fats stored in the body, not the gas-exchange surface. The pharynx is the throat that moves air toward the lungs, and bronchioles are small airways leading to the alveoli but are not the sacs themselves. So the structure responsible for gas exchange is the alveoli.